1. Field of the Invention
The present invention is in the field of gamma ray testing of geological formations. In particular, the invention determines the elemental composition and mineralogy of a formation from recorded spectra.
2. Description of the Related Art
Well logging systems have been utilized in hydrocarbon exploration for many years. Such systems provide data for use by geologists and petroleum engineers in making many determinations pertinent to hydrocarbon exploration. In particular, these systems provide data for subsurface structural mapping, defining the lithology of subsurface formations, identifying hydrocarbon-productive zones, and interpreting reservoir characteristics and contents. Many types of well logging systems exist which measure different formation parameters such as conductivity, travel time of acoustic waves within the formation and the like.
One class of systems seeks to measure incidence of nuclear particles on the well logging tool from the formation for purposes well known in the art. These systems take various forms, including those measuring natural gamma rays from the formation. Still other systems measure gamma rays in the formation caused by bursts of neutrons into the formation by a neutron source carried by the tool and pulsed at a preselected time interval.
In these nuclear well logging systems, reliance is made upon the physical phenomenon that the energies of gamma rays given off by nuclei resulting from natural radioactive decay or induced nuclear radiation are indicative of the presence of certain elements within the formation. In other words, formation elements will react in predictable ways, for example, when high-energy neutrons on the order of 14.2 MeV collide with the nuclei of the formation elements. Different elements in the formation may thus be identified from characteristic gamma ray energy levels released as a result of this neutron bombardment. Thus, the number of gamma rays at each energy level will be functionally related to the quantity of each element present in the formation, such as the element carbon, which is present in hydrocarbons. The presence of gamma rays at about 2.2 MeV energy level in the capture spectrum may for example, indicate the presence of hydrogen, whereas a predominance of gamma rays having energy levels of about 1.779 and 2.212 MeV in the inelastic spectrum, for example, may indicate the presence of silicon and aluminum respectively.
The measurement of neutron population decay rate is made cyclically. The neutron source is pulsed for 20-50 microseconds to create a neutron population. Neutrons leaving the pulsed source interact with the surrounding environment and are slowed down. In a well logging environment, collisions between the neutrons and nuclei of atoms in the surrounding fluid and formation act to slow these neutrons. Such collisions may impart sufficient energy to these atoms to leave them in an excited state, from which after a very short time gamma rays are emitted as the atom returns to a stable state. Such emitted gamma rays are labeled “inelastic gamma rays.” As the neutrons are slowed to the thermal state (less than 0.1 eV), they may be captured by atoms in the surrounding matter. Atoms capturing such neutrons are also caused to be in an excited state, and after a short time gamma rays are emitted as the atom returns to a stable state. Gamma rays emitted due to this neutron capture reaction are labeled capture gamma rays. In wireline well logging operations, as the neutron source is pulsed and the measurements made, the subsurface well logging instrument is continuously pulled up through the borehole. This makes it possible to evaluate formation characteristics over a range of depths.
Depending on the material composition of the earth formations proximal to the instrument, the thermal neutrons can be absorbed, or “captured”, at various rates by certain types of atomic nuclei in the earth formations. When one of these atomic nuclei captures a thermal neutron, it emits a gamma ray, which is referred to as a “capture gamma ray”.
Prior art methods exist for determining attributes of a formation from logging results. Reference is made to U.S. Pat. No. 4,712,424, to Herron, U.S. Pat. No. 4,394,574, to Grau et al., U.S. Pat. No. 4,390,783, to Grau for methods for analysis of nuclear data. Methods of decomposing obtained spectra into constituent spectra have been discussed, for instance, in SPE 7430 by Hertzog, Grau and Schweitzer(1987). The methods discussed in these papers correct an obtained inelastic spectrum by subtracting a background spectrum. Statistical analysis of obtained spectra is discussed by Roscoe et al., November-December, 1987, The Log Analyst. Reference is also made to paper E027 of the SPWLA by Pemper et al., SPE paper 4640 of Culver et al., and to “Hydraulic fracture evaluation with multiple radioactive tracers” by Pemper et al., (Geophysics, 1988).
Aluminum is one of the most important elements in lithology and mineral analysis. Many of the minerals encountered in petroleum exploration are associated with aluminum. Accurate determination of aluminum content can greatly improve formation mineral characterization process. Listed below are examples of aluminum rich minerals.    Feldspars: NaAlSI3O8 (albite),            CaAl2Si2O8 (anorthite),        KAlSi3O8 (orthoclase, microcline)            Clays: Al2Si2O5(OH)4 (Kaolinite),            (Na,Ca)(Al,Mg)6(Si4O10)3(OH)6—nH2O (montmorillonite)        (Fe,Mg,Al)6(Si,Al)4O10(OH)8(chlorite)        (K,H3O)(Al,Mg,Fe)2(Si,Al)4O10[(OH)2,(H2O)] (illite)            Micas KAl2(AlSi3O10)(F,OH)2 (muscovite)            K(Fe,Mg)3AlSi3O10(F,OH)2 (biotite)        (K,Na)(Fe,Al,Mg)2(Si,Al)4O10(OH)2 (glauconite)As noted in the Encyclopedia Britannica, identifying the various minerals, particularly those containing Aluminum, provides valuable information about mechanisms by which minerals of different sizes are transported and deposited, and also about the chemical conditions that permit precipitation of various authigenic minerals. As temperature and pressure increase with the progression of diagenesis, clay minerals in sediments under these circumstances change to those stable under given conditions. Therefore, certain sensitive clay minerals may serve as indicators for various stages of diagenesis. Typical examples are the crystallinity of illite, the polytypes of illite and chlorite, and the conversion of smectite to illite. Data indicate that smectite was transformed into illite through interstratified illite-smectite mineral phases as diagenetic processes advanced. Much detailed work has been devoted to the study of the conversion of smectite to illite in lower Cenozoic-Mesozoic sediments because such conversion appears to be closely related to oil-producing processes.        
One prior art method for identification of aluminum (Al) uses activation analysis. Al activation requires an additional activation source and gamma ray detectors and is discussed, for example, in SPE 16792 of Hertzog et al. The natural Al isotope, Al27, absorbs thermal neutrons and produces Al28 in an excited state.Al27+n→Al28→Si28*+β(2.24 minute half-life)→Si28+1.779 MeV photonDisadvantages of activation measurement are instrument complexity, low logging speed, and interference from activation of other elements in the formation such as manganese.
U.S. Pat. No. 5,471,057 to Herron describes a method for indirectly determining Al yield by modifying the iron gamma ray yield in thermal neutron capture measurement. This method is not accurate and suffers from low measurement sensitivity as aluminum has low thermal neutron absorption cross section of 0.23 barns. Although the Al capture spectrum resembles some of the iron spectrum features, the assumption that a constant correlation exists between aluminum and iron contents is often not true.
There is a need for a more complete analysis of the obtained gamma ray spectra. A separation of inelastic and capture gamma ray spectra yields a more complete understanding of the elemental composition of a geological structure. Consequently, an advantage can be obtained through a combined analysis of both inelastic and capture spectra in terms of their formation constituents. Such a method should give physically realistic analyses. The present invention fulfills this need.